[swift-dev] Strange DispatchTime bug

[David Zarzycki]

Hi Guillaume,

Without going into details, it looks like the Swift wrappers for dispatch_time_t are very, very broken. In short, dispatch_time_t (in the C world) is intentionally documented as an opaque type, but the Swift wrappers assume otherwise and jump to wrong conclusions about the implementation of the type. What this means in practice is that you should avoid the DispatchTime comparison APIs for now.

Dave

> On Aug 26, 2017, at 15:21, Guillaume Lessard via swift-dev <swift-dev at swift.org> wrote:
> 
> On Linux (Ubuntu 16.04) and swift distributions from 3.0.2 to the latest 4.0 snapshot, I’m seeing incorrect behaviour involving DispatchTime.distantFuture:
> (also: https://bugs.swift.org/browse/SR-5706)
> 
> ***
> import Dispatch
> 
> let now = DispatchTime.now()
> let future = DispatchTime.distantFuture
> assert(now != future, "this is not the distant future")
> 
> print(String(now.rawValue, radix: 16))                  // 1c20d6bbd3a (e.g.)
> print(String(future.rawValue, radix: 16))               // ffffffffffffffff
> 
> print(now < future)                                     // false (incorrect)
> print(future < now)                                     // false
> 
> print(now > future)                                     // false
> print(future > now)                                     // false (incorrect)
> 
> extension DispatchTime {
>  public static func compare(_ a: DispatchTime, happensBefore b: DispatchTime) -> Bool {
>    return a.rawValue < b.rawValue
>  }
> }
> 
> print(DispatchTime.compare(now, happensBefore: future)) // true
> print(DispatchTime.compare(future, happensBefore: now)) // false
> ***
> 
> Note that the static func I defined above is essentially the same as the less-than operator as defined in the libdispatch overlay: <https://github.com/apple/swift/blob/92f750aa3c3c4dce47eb55068850f3d1127b16bd/stdlib/public/SDK/Dispatch/Time.swift#L73>
> 
> Also, there is a test that should catch this problem:<https://github.com/apple/swift/blob/92f750aa3c3c4dce47eb55068850f3d1127b16bd/test/stdlib/Dispatch.swift#L104>
> 
> 
> Could this operator func somehow be called correctly during testing, but incorrectly at runtime?
> (In other words, I don’t see where the fix needs to be.)
> 
> Thanks,
> Guillaume Lessard
> 
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